U.S. Pat. No. 6,341,621 discloses a hydraulic tool that uses an O-ring as a check valve in the tool. The purpose of any check valve is to prevent fluid back flow. When using an O-ring as a check valve, there is a tipping point, e.g., a point that is reached where backpressure alone on the O-ring will close the check valve. This tipping point is dependent on several variables in the geometry of the valve seat.
There is a need to control the tipping point of an O-ring type check valve.